A wheel bearing apparatus is intended to freely rotationally support a wheel hub that mounts a wheel, via a double row rolling bearing. The wheel bearings are used for driving wheels and driven wheels. Due to structural reasons, the inner ring rotation type is used for driving wheels and both the inner ring rotation type and outer ring rotation type are used for driven wheels. There are four generation types of wheel bearing apparatus. A first generation type has a wheel bearing with double row angular contact ball bearing, etc. fit between a knuckle, forming part of a suspension apparatus, and the wheel hub. A second generation type has a body mounting flange or a wheel mounting flange directly formed on the outer circumference of an outer member of the wheel bearing apparatus. A third generation type has one inner raceway surface directly formed on the outer circumference of the wheel hub. A fourth generation type has the inner raceway surface directly formed on the outer circumference, respectively, of the wheel hub and an outer joint member of a constant velocity universal joint.
In these wheel bearings, a seal device is provided to prevent leakage of grease sealed inside the bearing and to prevent entry of rain water or dust from the outside into the bearing. One cause of premature damage of the seal device is that the seal device is used under severe environmental conditions, such as low temperatures and muddy water. It is believe that deterioration of the tightening force and followability of the sealing member occurs due to a reduction of elasticity under low environmental temperatures. This still occurs even if the sealing member is formed of improved material with low temperature properties. Thus, premature entry of muddy water into the inside of the bearing occurs. The sealability under low environmental temperatures can be improved by providing a garter spring on the sealing member. This maintains the tightening force and followability even under low temperature.
One example of a prior art seal device with a garter spring is shown in FIG. 7. This seal device 50 is a so-called pack-seal. It includes a slinger 53 and a sealing plate 54, disposed between an inner member 51 and an outer member 52, coaxially arranged with each other.
The slinger 53 has a cylindrical portion 53a and an annular portion. The cylindrical portion 53a is press-fit onto the outer circumference of the inner member 51. The annular portion 53b extends radially outward from the cylindrical portion 53a. The sealing plate 54 has a metal core 55 and a sealing member 56 integrally adhered to the metal core 55, via vulcanized adhesion. The metal core has a cylindrical portion 55a and an annular portion 55b. The cylindrical portion 55a is press-fit into the inner circumference of the outer member 52. The annular portion 55b extends radially inward from the cylindrical portion 55a. 
The sealing member 56 is formed from a rubber material. It includes a grease lip 56a, a dust lip 56b and an axial lip 56c. The grease lip 56a and dust lip 56b are in sliding contact with the cylindrical member 53a of the slinger 53. The axial lip 56c is in sliding contact with the annular portion 53b. 
The cylindrical portion 53a of the slinger 53 and the cylindrical portion 55a of the metal core 55 oppose one another in a radial direction, via a predetermined radial distance. The annular portion 53b of the slinger 53 and the annular portion 55b of the metal core 55 also oppose one another in an axial direction, via a predetermined axial distance. The former is positioned at an axially inboard-side and the latter is positioned at an axially outboard-side. An annular gap “S”, forming a labyrinth seal, is formed between a radially outermost end face 57 of the annular portion 53b of the slinger 53 and the cylindrical portion 55a of the metal core 55.
The radially outermost end face 57 of the annular portion 53b of the slinger 53 is precisely finished by machining. Thus, a dimension “φa2” of the outer diameter of the annular portion 53b of the slinger 53 and a dimension “e2” of the gap “S” have high precision. This enables finishing of the radially outermost end face 57 with a surface without any excess material, such as burrs, to improve sealability while reducing the gap “S” to as small as possible. See, JP2010-190323 A.